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An international team of scientists has discovered a distant star consisting of hydrogen and helium but very little metal  proving such stars may form in environments almost devoid of heavier elements.

Star HE 0107-5240 has the lowest abundance of heavier elements ever observed  only 1/200,000 of that of the Sun and 20 times less than any 'metal-deficient star' found previously. A group of astronomers from Germany, Sweden, Australia, Brazil and the USA announce their discovery in a letter to this week's Nature.

The finding provides researchers with a unique window towards the early stages of formation of the Milky Way. It also offers an opportunity to study stellar gas with a composition close to that produced during the Big Bang.

Stars made all the elements in the universe, except hydrogen, helium and a small amount of lithium. Early in the history of the Universe, stars began converting these three into heavier elements such as carbon, phosphorous and lead. Astronomers refer collectively to these as metals.

The Milky Way Galaxy formed from a gigantic cloud of gas, soon after the Big Bang. At first, this gas was presumably composed almost solely of hydrogen and helium atoms produced during this event.

However, once the first stars formed by contraction in that gas, many heavier elements were built up by nuclear processes in their interiors. As time passed, many of the stars of this and following stellar generations returned the processed matter to their surroundings when they reached the end of their lives. For example, during violent supernova explosions. This is how the interstellar gas in the Milky Way system has been continuously enriched with heavier elements.

Consequently, the early (or old) stars in the Milky Way differ from the younger stars by containing only very small amounts of such elements.

The question was, did any of these early stars still exist?

Over a period of more than 10 years, the ESO Schmidt Telescope in Chile took a large collection of photographic pictures of the southern sky. Using a large glass prism in the front of the telescope, stars and galaxies were imaged as a small spectrum, providing information about their type and composition.

When these were analsysed, about 8000 candidates for very metal-poor stars were found. These are now being scrutinised with many telescopes all over the world, in order to obtain a very detailed spectra. This allows astronomers to accurately determine their chemical composition.

HE 0107-5240 was observed in December 2001 with the UVES Spectograph on ESO's Very Large Telescope in Chile.

The star is about 10 thousand times fainter than the faintest stars that can be seen with the unaided eye. It is located in the direction of the constellation Phoenix and is about 36,000 light years away.

The finding clearly demonstrates that stars with masses slightly less than the Sun can form from very metal-poor gas.